This invention relates to communication receivers and more particularly to a very fast automatic gain control circuit.
In the past automatic gain control (AGC) circuits have been used in almost all communication receivers. With audio (voice) systems the response time of the AGC has been 30 milli-seconds or more. This response time is acceptable for voice communication systems, but is not fast enough for use in high speed communication systems. For high speed communication receiver systems, a very fast AGC response time is required to prevent loss of information.
In an article by A. Uhlir, Jr., "The Potential of Semiconductor Diodes in High-Frequency Communications", Proc. IEEE, vol. 46, pp. 1099-1115, June 1958, it was recognized that a p-i-n diode limiter circuit can be used to maintain a fairly constant output amplitude from a variable-frequency oscillator; the accuracy might be improved by a feedback circuit that adjusts the bias voltage in accordance with a power monitor.
Further, L. J. Giacoletto, Electronics Designer's Handbook, (2nd Ed. 1977), pp 24-163-24-167 teaches the use of p-i-n diodes as variable attenuators at very high frequencies. The field of applications of p-i-n diodes ranges from about 10 MHz to a few GHz and from lumped AGC circuits to constant-impedance attenuators in distributed structures and antenna switches. A fairly constant operation can be obtained over a wide frequency range, say from 50 MHz to 1 GHZ; the minimum attenuation is only approximately 1 dB, and the increase in noise factor is small.
The major problem with a fast AGC is instability. The instability in a wide dynamic range AGC is mainly due to the size of the input signal which lasts in duration a time equal to the response time.